1. Field of the Invention
The present invention relates to a surface acoustic wave device such as surface acoustic wave (SAW) filter, surface acoustic wave resonator and surface acoustic wave guide, and more particularly relates to a surface acoustic filter with low loss utilizing a natural single-phase unidirectional transducer (NSPUDT) property.
2. Description of the Related Art
Heretofore, surface acoustic wave devices have been desired to have large electro-mechanical coupling coefficient K.sup.2, small power flow angle PFA, small temperature coefficient of delay time TCD and so on. Furthermore, recent electronic devices are required to be small in size, light in weight and economical in power consumption.
There has been widely used a transversal type surface acoustic wave filter including a substrate having a piezoelectric property, a transmitter side transducer having interdigitally arranged positive and negative electrode fingers, which are connected to two opposite phase output terminals of a single-phase signal source, and a receiver side transducer having interdigitally arranged positive and negative electrode fingers. Such a transversal type surface acoustic wave filter has been used for extracting a desired signal component within a given frequency range.
In such a surface acoustic wave filter, it has been required to reduce an insertion loss and to suppress undesired ripple components within a given frequency range. Since usual interdigital type electrode structures have a bidirectional property, the insertion loss could not be reduced smaller than a theoretical insertion loss of 6 dB. This insertion loss of 6 dB is too large to realize a SAW filter having a desired property.
In order to mitigate such a drawback, there has been proposed a multi-transducer system, in which on both sides of each of a plurality of transmitter side transducers, there is arranged a pair of receiver side transducers. In a surface acoustic wave filter of such a multi-transducer system, it is possible to reduce the insertion loss to about 1.5-2.0 dB. However, in the multi-transducer system, the control of a plurality of transducers is very difficult and good phase and frequency properties could not be attained. Moreover, it is quite difficult to manufacture such a multi-transducer system device. In order to improve the performance of the surface acoustic wave device, it is also very important to flatten a phase characteristic, to suppress pass band ripples and to reduce a suppression band.
In order to meet the above mentioned requirements, it has been proposed to use a unidirectional transducer that can realize a very small insertion loss which is theoretically lower than 1 dB and has excellent phase and frequency characteristics. There have been proposed various types of the unidirectional transducers. They may be roughly classified into:
(a) multi-phase type unidirectional transducer, and PA1 (b) single-phase type unidirectional transducer. PA1 a substrate made of a single crystal of doubly rotated Y-cut (.alpha.Y-.theta.X cut) Langasite (La.sub.3 Ga.sub.5 SiO.sub.14) and having a surface, a rotation angle .alpha. from the Y axis being substantially 50.degree..+-.5.degree. and a rotation angle .theta. from the X axis being substantially 19.degree.-+.degree.; PA1 a first electrode structure formed on said surface of the substrate such that a natural single-phase unidirectional transducer property is revealed together with an anisotropy of the substrate; and PA1 a second electrode structure formed on said surface of the substrate such that the natural single-phase unidirectional transducer property due to the anisotropy of the substrate is cancelled. PA1 a substrate made of a single crystal of doubly rotated Y-cut (.alpha.Y-.theta.X cut) Langasite (La.sub.3 Ga.sub.5 SiO.sub.14) and having a surface, a rotation angle .alpha. from the Y axis being substantially 50.degree..+-.5.degree. and a rotation angle .theta. from the X axis being substantially 19.degree.-30.degree.; PA1 a first electrode structure formed on said surface of the substrate such that a natural single-phase unidirectional transducer property is revealed together with an anisotropy of the substrate; and PA1 a second electrode structure formed on said surface of the substrate such that a directionality of the natural single-phase unidirectional transducer property due to the anisotropy of the substrate is reversed.
As the latter single-phase type unidirectional transducer, there have been proposed various types. For instance, there has been proposed a single-phase unidirectional transducer utilizing an asymmetry of electrode structure and an internal reflection due to the mass load effect, a reflection bank type unidirectional transducer having a reflection bank between exiting electrodes, a single-phase unidirectional transducer utilizing a reflection by floating electrodes, and a natural single-phase unidirectional transducer utilizing anisotropic property of a substrate. In surface acoustic wave devices including these unidirectional transducers, the directionality is realized by making an exciting wave and a reflected wave to be in-phase in a wave propagating direction and to be in opposite phase in an opposite direction.
Heretofore, substrates of surface acoustic wave devices are made of single crystals of quartz, lithium tantalate (LiTaO.sub.3) and lithium niobate (LiNbO.sub.3). In U.S. Pat. No. 5,698,927, there is proposed the use of lithium tetraborate (LiB.sub.4 O.sub.7) single crystal as a substrate revealing the natural single-phase unidirectional transducer operation. However, the known single crystals are not optimal for the surface acoustic wave device, and it is particularly difficult to obtain a surface acoustic wave filter having desired characteristics.
For instance, in order to realize a surface acoustic wave resonator, it is required that the electro-mechanical coupling coefficient K.sup.2 be large, the reflection be large, the power flow angle PFA be small and the temperature coefficient of delay time TCD be small. However, the electro-mechanical coupling coefficient K.sup.2 of quartz is about 0.2 and the reflection of quartz is small. Therefore, it is rather difficult to realize a surface acoustic wave resonator having desired characteristics by using the quartz substrate.
A substrate for the surface acoustic wave filter is required to have a large electro-mechanical coupling coefficient K.sup.2, a small reflection, a small power flow angle PFA and a small temperature coefficient of delay time TCD. However, the use of a piezoelectric substrate having such characteristics has not been proposed.
In the above mentioned known single-phase unidirectional transducer, except for the natural single-phase unidirectional transducer, the electrode structure has become very complicated. Particularly, the distance between adjacent electrode edges and the width of an electrode have to be smaller than .lambda./4. Upon increasing the operation frequency, the distance and width should be much smaller, and it is quite difficult to manufacture electrodes of such small distance and width.
In order to mitigate the above mentioned drawbacks, there has been proposed a natural single-phase unidirectional transducer (NSPUDT), in which the unidirectionality is realized by utilizing the anisotropy of the piezoelectric substrate although the normal type electrode structure having an electrode edge distance of .lambda./4 and an electrode width of .lambda./4 is used. It is known that quartz substrate, LiNbO.sub.3 substrate and LiTaO.sub.3 substrate, show the NSPUDT operation due to anisotropy. However, in these known piezo-electric substrates, the electro-mechanical coupling coefficient K.sup.2 is small, the temperature coefficient of delay time TCD is not zero, the power flow angle PFA is not zero, and a directionality reversed electrode structure could not be realized easily. Due to these problems, it is very difficult to realize an ideal surface acoustic wave device.
Furthermore, in order to obtain a surface acoustic wave device which is small in size, light in weight and low in electric power consumption, it is advantageous to decrease the propagating velocity of a surface acoustic wave. However, the above-mentioned piezoelectric substrates have a relatively high propagating velocity which is not lower than 3000 m/sec. Therefore, the known piezoelectric substrates cannot produce a surface acoustic wave device which is small in size, light in weight and low in electric power consumption.
Recently, a piezoelectric substrate made of Langasite (La.sub.3 Ga.sub.5 SiO.sub.14) single crystal has been proposed for the surface acoustic wave device. It has been reported that the Langasite single crystal belongs to the trigonal system to which the quartz crystal also belongs, and has a higher electro-chemical coupling coefficient K.sup.2 and a higher phase transition temperature than the quartz crystal ("The 17.sup.th ULTRASONIC SYMPOSIUM Preliminary Theses", page 305, 1996). In the same publication, page 306, there is also described that in an X-cut Langasite single crystal, the temperature dependency becomes almost zero when a rotation angle .theta. from the Y axis is set to a value within a range of 10.degree.-20.degree.. Further, in "1996 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM, FILTER AND RESONATOR USING LANGASITE", pp. 379-382, 1996, it has been reported that a Langasite single crystal is used as a substrate for a bulk resonator. It should be noted that the bulk resonator is quite different from the surface acoustic wave device. Moreover, in "Japan Journal of Applied Physics", Vol. 37, part 1, No. 5B, page 2909, 1998 as well as in "27.sup.th EM SYMPOSIUM Preliminary Theses", 1998, page 125, it has been reported that a doubly rotated Y-cut (.alpha.X-.theta.Y cut) Langasite single crystal substrate having a rotation angle .alpha. from the X axis of 50.degree. and a rotation angle .theta. from the Y axis of 22.5.degree.-24.degree. has a superior temperature property which is compatible with the ST-X cut quartz crystal substrate.
As stated above, several reports have been issued about the velocity of the surface acoustic wave, electro-mechanical coupling coefficient K.sup.2 and temperature dependency for the Langasite single crystal substrates. However, no one has reported the electrode reflection property. Particularly, the above publications do not mention at all that the Langasite single crystal has the unidirectionality due to the anisotropy, i.e. NSPUDT property.
The inventors have reported in "Lecture Theses of The Acoustical Society of Japan", 1-8-22, page 997, 1997 that the Langasite single crystal substrate can be advantageously used for surface acoustic wave devices. In this publication, it is reported that the doubly rotated Y-cut Langasite single crystal substrate having a rotation angle .alpha.=0.degree.-30.degree. and a rotation angle .theta.=0.degree.-20.degree., shows a good NSPUDT property. After various experiments and analyses, the inventors have found that the power flow angle PFA of such a Langasite single crystal substrate is not zero, and therefore this known Langasite single crystal substrate could not produce a surface acoustic wave device having an optimal performance.